Nanotube Structures, Materials, and Methods

ABSTRACT

Nanotube structures and methods for forming nanotube structures are disclosed. The methods include forming nanotubes such that they are associated with a surface of a substrate and compressing at least a portion of the nanotubes. In some embodiments, the nanotubes may be dimensionally constrained in one direction while being compressed in another direction. Compressing at least a portion of the nanotubes may comprise stamping an impression into a surface of the nanotubes, at least a portion of which is retained when the stamp is removed. In some embodiments, the nanotubes may be aligned with respect to one another and to the surface of the substrate and may extend in a direction that is, for example, normal to the substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 11/897,893, filed Aug. 30, 2007, which claims priority to, andthe benefit of, U.S. Provisional Application Nos. 60/841,266, filed Aug.30, 2006; 60/876,336, filed Dec. 21, 2006; and 60/923,904, filed Apr.17, 2007. This application also claims priority to, and the benefit of,U.S. Provisional Patent Application No. 61/066,647, filed Feb. 22, 2008.The entire contents of all of those applications are incorporated byreference herein in their entireties.

STATEMENT REGARDING FEDERALLY FUNDED RESEARCH AND DEVELOPMENT

This invention was made with United States Government support under SBIRContract # 0422198 from the National Science Foundation. The UnitedStates has certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to the field of materials science, andmore particularly to nanotube structures, materials, and methods.

2. Description of Related Art

By most accounts, synthetic nanotubes have been described in thescientific literature since 1991, see Iijima, “Helical microtubules ofgraphitic carbon”, Nature, vol. 354, Nov. 7, 1991. Iijima described thenanotubes as a product of arc-evaporation synthesis of fullerenes.Scientists have since determined that carbon nanotubes have unusual andcommercially valuable physical characteristics, and their potential usein many different applications has attracted much attention. Forexample, single-wall carbon nanotubes have high-current density and lowcapacitance characteristics.

Research has shown that single-walled nanotubes have the highestreversible capacity of any carbon material for use in lithium ionbatteries. Carbon nanotubes also have applications in a variety of fuelcell components. They have a number of properties, including highsurface area and thermal conductivity, which make them useful aselectrode catalyst supports in PEM fuel cells. Because of their highelectrical conductivity, they may also be used in gas diffusion layers,as well as current collectors. Carbon nanotubes' high strength andtoughness-to-weight characteristics may also prove valuable as part ofcomposite components in fuel cells that are deployed in transportapplications, where durability is extremely important.

The unique properties of nanotubes, especially carbon nanotubes, haveresulted in attempts to use nanotubes in a variety of differentapplications. In many of these applications, a body of relatively densenanotubes is desired. For example, an application using nanotubes tostore a chemical or electrical species may have a constraint on themaximum size of the storage container, and the user desires the maximumstorage within this volume. Thermal management applications may alsorequire a high density of nanotubes, and these latter applications mayalso require an aligned body of nanotubes. In these and otherapplications, a maximum density of nanotubes is desired.

However, synthesis methods for nanotubes often result in arrays or othersets of nanotubes having a relatively low packing density of nanotubesin the array or set (e.g. for carbon nanotubes, sometimes below 1% ofthe theoretical density of graphite). For chemical vapor deposition(CVD) growth of nanotubes on a substrate, a low nanotube density mayeven enhance growth rates by allowing gases to easily pass through thetubes to the tube/substrate interface where growth occurs. Although lowdensities may be helpful during nanotube formation, many applicationsrequire more densely packed nanotubes.

SUMMARY OF THE INVENTION

One aspect of the invention relates to methods for forming nanotubestructures. The methods comprise forming nanotubes such that they areassociated with the surface of a substrate and then applying acompressive force to at least a portion of the nanotubes in at least onedirection.

Another aspect of the invention relates to a method for making ananotube structure. The method comprises forming nanotubes such thatthey are associated with a surface of a substrate, impressing a stamphaving a stamp surface upon at least a portion of the nanotubes to makean impression in at least the portion of the nanotubes, and removing thestamp from the portion of the nanotubes. The portion of the nanotubesretains at least a portion of the impression.

Yet another aspect of the invention relates to a nanotube structureproduced by a process comprising forming nanotubes on a surface of asubstrate, and applying a compressive force to at least a portion of thenanotubes to form the nanotube structure. The resulting nanotubestructure is denser and smaller in at least one dimension than thenanotubes.

Other aspects, features, and advantages of the invention will be setforth in the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with respect to the following drawingfigures, in which like numerals represent like structures throughout thefigures, and in which:

FIG. 1 is a flowchart generally illustrating a method for forming afreestanding nanotube object according to an exemplary embodiment of theinvention;

FIGS. 2A-2C schematically show exemplary tasks for providing a substrateaccording to an exemplary embodiment of the invention;

FIG. 3 schematically shows an array of substantially aligned nanotubeson a surface of a substrate according to an exemplary embodiment of theinvention;

FIG. 4A schematically shows a portion of an array separated from thesurface according to an exemplary embodiment of the invention;

FIG. 4B schematically illustrates a method for mechanically separatingand densifying a portion of an array according to an exemplaryembodiment of the invention;

FIG. 5 schematically illustrates a process for densifying an array ofnanotubes according to an exemplary embodiment of the invention;

FIG. 6 schematically illustrates tasks of a method for partiallydensifying a wet nanotube array prior to drying, according to anexemplary embodiment of the invention;

FIGS. 7A-7D show examples of assemblages constructed from densifiednanotube arrays, according to exemplary embodiments of the invention;

FIGS. 8A and 8B schematically illustrate another exemplary method forcreating an assemblage, according to an exemplary embodiment of theinvention;

FIG. 9 shows a graphical representation of nanotube diameter, nanotubewall thickness, and nanotube density for four exemplary catalysts;

FIGS. 10A-D are scanning electron microscope images of as-grownvertically aligned nanotubes at magnification levels of 5,000×, 10,000×,50,000× and 100,000×, respectively;

FIGS. 11A-D are scanning electron microscope images of nanotubes afterdensification via capillary forces at the same magnification levels asthe nanotubes of FIGS. 10A-D;

FIGS. 12A-D are scanning electron microscope images of nanotubes furthercompressed with mechanical force at the same magnification levels as thenanotubes of FIGS. 11A-D;

FIG. 13 is a schematic side elevational view illustrating theapplication of multiple mechanical forces to a nanotube sample;

FIG. 14 is a schematic side elevational view illustrating a stampingprocess on a nanotube sample; and

FIGS. 15-17 are illustrations of various types of stamps that may beused on nanotubes in embodiments of the invention.

DETAILED DESCRIPTION

The specification provides methods for forming freestanding objectsformed primarily from aligned carbon nanotubes, as well as the objectsmade by these methods. In these methods, arrays of generally alignedcarbon nanotubes are first synthesized and then densified, maintainingthe aligned arrangement. The densified arrays can take the form of thinstrips which can be joined together, for example by lamination, to formlarger objects of arbitrary size. These objects can be further cut orotherwise machined to desired dimensions and shapes.

FIG. 1 shows a flowchart 100 that provides an overview of an exemplaryembodiment of the invention. In a Task 110, a suitable substrate isprovided for the growth of nanotubes. In a Task 120, one or morenanotube arrays are grown on the surface of the substrate. Task 130comprises separating at least a portion of an array from the substrate,and Task 140 comprises densifying the separated portion. In general, thesteps of providing the substrate and growing the one or more arrays ofnanotubes are performed prior to the steps of separation anddensification. In different embodiments, the step of densificationfollows the step of separation, the step of densification starts beforethe step of separation is completed, or the two steps are performedsimultaneously. In some embodiments, which will be described below inmore detail, densification may be performed without first separating thearray from the substrate.

FIGS. 2A-2C illustrate exemplary steps of Task 110 (FIG. 1) of providingthe substrate. In FIG. 2A, Substrate 210 is a suitable substrate for thegrowth of nanotubes. For the purposes of Task 120 (FIG. 1), in which ahigh temperature process such as chemical vapor deposition (CVD) is usedto grow one or more arrays of refractory nanotubes such as carbonnanotubes, Substrate 210 can be composed of a refractory material suchas Si, SiO₂, or Al₂O₃. However, nanotubes may be formed by processesother than CVD, and in certain embodiments of the invention, thesubstrate may comprise a material selected from the list includingsilicon; silica; carbon; graphite; diamond; metal; steel; stainlesssteel; gold; silver; a chalcogenide; polymer; silicone; glass; quartz;ceramic; and, piezoelectric material.

It will be appreciated that although Substrate 210 is shown in FIGS.2A-2C as flat, the Substrate 210 is not limited to being flat and mayhave whatever profile is desired in other embodiments.

As shown in FIG. 2B, Substrate 210 is provided with an Active Surface220 on which the one or more arrays of nanotubes will be grown in Task120. The Active Surface 220 can cover the entire surface of theSubstrate 210 as shown, or can be limited to portions of the surface ofthe Substrate 210, as discussed below in connection with FIG. 2C.Because nanotube growth can be sensitive to the composition andmorphology of the Active Surface 220, preparing the Active Surface 220can optionally include cleaning the surface of the Substrate 210 and/orproviding a catalyst layer on the surface of the Substrate 210 toenhance growth. An exemplary cleaning procedure for silicon substratesincludes immersion for 10 minutes in a 4:1 bath of H₂SO₄/H₂O₂,maintained at 120° C. The Substrate 210 is then rinsed in water andimmersed for 10 minutes in a 5:1:1 bath of H₂O/H₂O₂/HCl, maintained at90° C. The Substrate 210 is then rinsed in water and immersed for 1minute in a 50:1 HF:H₂O room temperature bath. The Substrate 210 is thenrinsed in water and spun dry.

Exemplary catalysts for carbon nanotube synthesis are well known andinclude Fe, Co, Ni, Mo and oxides thereof. In some embodiments,providing the catalyst layer on the surface of the Substrate 210includes creating small (e.g. <100 nm) catalyst particles on the ActiveSurface 220. Techniques such as physical vapor deposition (PVD) followedby annealing can be used to create these particles. U.S. Pat. No.7,235,159 “Methods for Producing and Using Catalytic Substrates forCarbon Nanotube Growth” discloses several methods, and is incorporatedby reference herein in its entirety.

In some aspects that include providing the catalyst layer, the ActiveSurface 220 is enhanced by forming one or more interfacial layersbetween the Substrate 210 and the catalyst layer. For example, aninterfacial layer can comprise about a 10-150 nm thick Al₂O₃ layerbetween the catalyst layer and the surface of the Substrate 210. Anotherinterfacial layer can comprise an approximately 500 nm thick SiO₂ layerdisposed between the Al₂O₃ layer and the Substrate 210.

In some aspects, providing the catalyst layer includes depositing a thinlayer of a catalyst material on the Substrate 210 through the use ofelectron beam evaporation followed by annealing the Substrate 210.Exemplary catalysts can comprise Fe, Co, Ni, Mo, Ru and combinationsthereof. Suitable thicknesses of the deposited layer are between 0.1 nmand 5 nm, and preferably between 1 nm and 3 nm.

Providing the catalyst layer, in some embodiments, can comprisesequentially depositing multiple layers of catalyst, optionally with anintermediary processing step between layers to effect a chemical orphysical change in the initially deposited layer or layers prior to thedeposition of a subsequent layer. Examples of such a multilayerdeposition process include depositing a 1 nm Fe layer followed by ananneal and then by depositing another 1 nm Fe layer (1 nm Fe/anneal/1 nmFe); 1 nm Fe/anneal/1 nm Co; and 1 nm Co/anneal/1 nm Fe. An appropriateanneal may be performed at temperatures between about 600 and 900° C.,and more preferably between about 700 and 800° C. For an annealing tubefurnace having a 6″ diameter tube, an appropriate ambient comprises 2.5standard liters per minute of ultra high purity argon, with a ramp rateof 15° C./minute and virtually no dwell time at the desired maximumannealing temperature.

The Active Surface 220 may optionally be patterned; creating Boundaries230 that define bounded Regions 240, as shown in FIG. 2C. A Boundary 230may be straight, curved, angled, continuous, discontinuous, regular orirregular. Although shown as a line in FIG. 2C, the Boundary 230 canhave an appreciable width to separate one Region 240 from the next. Anexemplary Region 240 is rectangular, as shown, but is not so limited.

Nanotube growth on or within a Boundary 230 may be prevented orminimized by not applying a catalyst to the surface of the Substrate 210on the Boundary 230. In some aspects, the Substrate 210 can be masked,then subjected to a line of sight deposition technique (e.g. PVD) fordeposition of a catalyst. Here, the masked regions do not receivedeposited catalyst, while the regions exposed to the catalyst depositionbecome the Regions 240 that comprise the Active Surface 220. Contactmasks, lithography, and other masking methods can be used to demarcateBoundaries 230.

An exemplary photolithography method includes coating a silicon waferwith a 1 μm positive photoresist layer, baking the resist, masking theresist using an appropriate mask, exposing the resist, developing theresist, dissolving the exposed portion (or unexposed, if using anegative resist), and optionally inspecting the wafer. Followingcatalyst deposition, the remaining resist is lifted off (e.g. via anacetone soak, optionally including surface swabbing) followed by anacetone rinse and an isopropanol rinse, followed by drying in nitrogen.

For the purposes of this specification, a Region 240 is any contiguousarea on the surface of the Substrate 210 where nanotubes are intended togrow in Task 120. If no Boundaries 230 are present, the entire surfaceof the Substrate 210 will define a single unbounded Region 240. IfBoundaries 230 are present, multiple discrete Regions 240 will exist onthe surface of the Substrate 210. The number, size, shape and otheraspects of each Region 240 may depend on the final application of thenanotubes fabricated thereon. In addition to the methods describedabove, Boundaries 230 can also be created after the formation of theActive Surface 220 (e.g. by masking followed by etching or ablating, orby targeted ablating of appropriate areas of the Active Surface 220).

Each Region 240 of the Active Surface 220 can be characterized by atleast one Length 250 that is the smallest lateral dimension of theRegion 240. As will be discussed later, Length 250 may correspond to thesmallest lateral dimension of a nanotube array grown on that Region 240.

Methods for growing nanotubes, especially carbon nanotubes, are wellknown. For the purposes of the present invention, growth conditions thatyield substantially aligned nanotubes are utilized for Task 120 (FIG.1). For certain densification processes in Task 140 (FIG. 1), discussedbelow, in which separation of the nanotubes includes etching, it may beadvantageous to choose a “base growth” method for growing the nanotubes(in which the nanotubes grow from the point of attachment to thecatalyst) and to choose a catalyst that can be readily etched. Exemplarythermal CVD growth conditions for growing carbon nanotubes in a 1″ tubefurnace include a temperature in the range of about 700-800° C.Substrates may be initially heated in an inert atmosphere (e.g. argon),then exposed to a deposition atmosphere comprising, for example, ahydrocarbon component. An exemplary deposition atmosphere includes 0.1standard liters per minute (SLM) ethylene, 0.4 SLM hydrogen, and watervapor. An appropriate water vapor concentration may be created bybubbling 0.1 SLM of argon through a water bubbler at ambienttemperature. Typical growth times are between 5 minutes and 100 minutes.

FIG. 3 schematically shows an exemplary nanotube array that may be grownaccording to exemplary embodiments of Task 120 (FIG. 1). As in FIGS.2A-2C, the Substrate 210 includes a Region 240 having a Length 250. InTask 120 an Array 310 of nanotubes is grown on the Region 240 to anArray Height 315. Nanotubes in the Array 310 are connected to the ActiveSurface 220 at an Interface 320. It may be advantageous to grow theArray 310 to a sufficient Height 315 that the ratio of the Height 315 tothe Length 250 is greater than 1:1. For thermal CVD growth of carbonnanotubes as described herein, 30 minutes of growth at 800° C. can yieldan Array 310 having a Height 315 of approximately 1 mm or greater.

FIG. 4A schematically shows an exemplary result of separating a Portion410 of the Array 310 from the Substrate 210 after Task 130 (FIG. 1). Asshown, the Portion 410 can include the entire Array 310, while in someaspects the Portion 410 comprises some segment of the Array 310. Forexample, it may be advantageous to release all of the Array 310 exceptfor one or more small attached sections at the edges, in order to keepthe Array 310 loosely connected to the Substrate 210 for furtherprocessing.

In some embodiments, Task 130 is performed by introducing a suitableSeparation Atmosphere 420 to the Substrate 210 and Array 310, where theSeparation Atmosphere 420 is capable of etching the Interface 320 torelease the Portion 410 from Substrate 210. Advantageously, in someembodiments Task 120 of growing the Array 310 and Task 130 of releasingat least the portion 410 of the Array 310 can be performed in the samereaction vessel. For example, the Separation Atmosphere 420 can beintroduced into the reaction vessel (e.g., a tube furnace) immediatelyfollowing the deposition atmosphere. An exemplary Separation Atmosphere420 includes 0.5 SLM hydrogen and water carried on 0.1 SLM argon bubbledthrough a water bubbler at ambient temperature.

FIG. 4B schematically illustrates a method for concurrently separatingand densifying a Portion 410 of an Array 310. In this example, thePortion 410 comprises the entire Array 310 which is subjected to Forces430, 440, and 450 by Blocks 460, 470, and 480, respectively. Forexample, Force 430 is used to move the Block 460 in the direction 485,as shown, to mechanically shear the nanotubes of the Array 310 off ofthe Substrate 210. As the nanotubes of the Array 310 are separated fromthe Substrate 210 and densified through compaction against Block 480,the Force 450 increases against the Array 310. The Block 470 also exertsthe Force 440 against the top of the Array 310 in order to resistbuckling of the Array 310 in response to the Forces 430 and 440. Whilein the illustrated example the Block 480 is fixed and exerts the Force450 reactively in response to the compaction of nanotubes against theBlock 480, in some embodiments Blocks 460 and 480 are moved towards oneanother to compact the Array 310 from both sides.

FIG. 5 schematically illustrates, for Task 140 (FIG. 1), an exemplarymethod for densifying a Portion 410 that has been previously separatedfrom the Substrate 210. In this method, the Portion 410 is first exposedto a Wetting Environment 510 to create a Wet Portion 520 that issubsequently dried to create a Densified Portion 530. When allowed todry, capillary forces between nanotubes in the Wet Portion 520 draw thenanotubes closer together as the drying progresses.

The Wetting Environment 510 can comprise a wetting fluid and a type ofexposure. For instance, the type of exposure can be immersion in thewetting fluid, exposure to a vapor including the wetting fluid, orexposure to a mist of the wetting fluid. A wetting fluid having at leastsome nonpolarity may be advantageous, and isopropyl alcohol (andsolutions thereof) is a suitable example. Other suitable wetting fluidsinclude water, xylene, acetone, methanol, and ethanol. The choice of awetting fluid may also be partially influenced by desired dryingkinetics. Fluids with particularly low vapor pressures may dry tooslowly; fluids with particularly high vapor pressures may dry tooquickly (at a given drying condition of pressure and temperature).

One or more surfactants may also be added to the Wetting Environment510. Surfactants may increase the wetting of the nanotubes by aparticular fluid. Surfactants may also substantially adhere to thenanotubes, and in some cases may create a degree of steric separationbetween tubes (e.g. surfactant molecules prevent two tubes fromapproaching closer than a certain distance). In certain aspects, thissteric separation may be used to control a final density by maintaininga minimum separation between tubes or groups of tubes. Thus, a desireddensity may be achieved by balancing the compressive forces of thedensification process with repulsive forces between tubes (e.g. by asurfactant), allowing a user to tailor the density to a targetapplication. Certain embodiments may result in nanotube bodies havingfinal mass densities between 2% and 97%, preferably between 5% and 90%,more preferably between 10% and 80%, and still more preferably between15% and 70%. Sodium dodecylbenzene sulfonate can be a suitablesurfactant for creating these steric forces.

As noted above, Portion 410 can be exposed directly to a liquid, forexample, by immersion. In these embodiments the Portion 410 rapidlybecomes saturated with the wetting fluid. Potion 410 can also be exposedto a mist or vapor such that the exposure begins gradually and increasesuntil the Portion 410 is sufficiently wet. In some cases, completesaturation of Portion 410 (i.e. substantially filling all intertubularspace with the wetting fluid) may not be necessary.

As shown in FIG. 5, Wet Portion 520 is exposed to a Drying Atmosphere540 to produce the Densified Portion 530. Where the wetting fluid isisopropyl alcohol, for example, a Drying Atmosphere 540 comprising airat ambient temperature and pressure can produce the Densified Portion530 within a few minutes. In some embodiments, drying the Wet Portion520 results in the Densified Portion 530 having a Length 550 that issmaller than the corresponding Length 250 of the Portion 410 prior todensification. By gently wetting and drying the Portion 410, alignmentof the nanotubes can be preserved to enhance densification, resulting ina Length 550 substantially smaller than the corresponding Length 250 ofthe Portion 410 prior to densification.

FIG. 5 shows the Densified Portion 530 disposed on a Drying Substrate560, but it will be appreciated that the Drying Substrate 560 is notessential in some embodiments. For example, the Wet Portion 520 can besuspended in the Drying Atmosphere 540 by one end and allowed todensify. In some embodiments, however, it is advantageous to supportand/or mold the Wet Portion 520 while disposed in the WettingEnvironment 510 and/or the Drying Atmosphere 540. For instance, the WetPortion 520 can be removed from the Wetting Environment 510 using asubstrate, form, mandrel, mold or other support, which may also be usedto mold, support and/or constrain Wet Portion 520 during the subsequentexposure to the Drying Atmosphere 540. The orientation of such a support(e.g. Drying Substrate 560) with respect to a supported portion can besuch that the nanotubes are aligned substantially parallel to thesupport, substantially perpendicular to the support, or in any desiredorientation. One or many portions may be supported by a single support,and the degree of alignment or randomness among portions can be tailoredto a desired application. Appropriate materials for such a supportinclude fused silica, Teflon® (polytetrafluoroethylene), silicon, andsilicone.

FIG. 5 shows an example in which the support comprises the DryingSubstrate 560. It may be advantageous to fabricate the Drying Substrate560 from an elastic material (e.g. rubber, latex, or silicone), whichallows the Drying Substrate 560 to be stretched. By stretching DryingSubstrate 560 prior to contact with the Wet Portion 520, Wet Portion 520may be placed or molded onto the stretched Drying Substrate 560.Subsequently, releasing or relaxing the stretched Drying Substrate 560during exposure to the Wetting Environment 510 and/or the DryingAtmosphere 540, or even after drying, will cause the Drying Substrate560 to contract which may enhance the densification of the portionattached thereto. In some of these embodiments, the Wet Portion 520 isoriented on the Drying Substrate 560 such that the direction ofalignment of the nanotubes is normal to the surface of the DryingSubstrate 560, whereas in other embodiments the orientation of the WetPortion 520 is as shown in FIG. 5.

FIG. 6 illustrates steps of a method for partially densifying the WetPortion 520 prior to drying. In this example, Wet Portion 520, whileexposed to Wetting Environment 510, is disposed between Blocks 610 and620 as shown. Blocks 610 and 620 are then actuated to apply Forces 630and 640 to compress and shear the Wet Portion 520. In this way thenanotubes of the Wet Portion 520 “lay down” with respect to theirinitial alignment as illustrated while maintaining the substantiallyparallel alignment that existed prior to the application of Forces 630and 640. While the Forces 630 and 640 already include a compressivecomponent, further densification can be achieved by applying additionalcompression, without the shear component, after the application ofForces 630 and 640.

Following the mechanical manipulation shown in FIG. 6, the nanotubes areexposed to the Drying Atmosphere 540, discussed above, to create theDensified Portion 530. One or both of Blocks 610 and/or 620 can beremoved prior to the exposure to the Drying Atmosphere 540. Optionally,either or both of the Blocks 610 and 620 can comprise a shaped surfaceagainst which the Wet Portion 520 can be molded during the processillustrated by FIG. 6 and/or during the subsequent drying process.

FIGS. 7A-7D show examples of freestanding assemblages constructed fromDensified Portions 530 according to various embodiments. These examplesare illustrative, and not meant to be limiting.

Assemblage 710, shown in FIG. 7A, comprises multiple Densified Portions530, arranged in an ostensibly random fashion. Although FIG. 7A showsthe Densified Portions 530 each having a direction of nanotube alignmentthat is in the plane of the drawing page, it will be appreciated thatthe direction of nanotube alignment for the multiple Densified Portions530 can be randomly oriented in three dimensions as well. In someinstances, Assemblage 710 may be constrained within a package such as acan or an envelope. In these embodiments, the multiple DensifiedPortions 530 can be loosely arranged or packed for greater density. Insome cases the package can be the support discussed above with respectto FIG. 5.

FIG. 7B shows an Assemblage 720 that comprises multiple DensifiedPortions 530 arranged “end to end” such that the longitudinal directionof the nanotubes (the direction of nanotube alignment) is substantiallymaintained throughout the Assemblage 720. In some embodiments, themultiple Densified Portions 530 are joined together by a bonding agentsuch as a glue or an adhesive. Exemplary glues and adhesives includecyanoacrylates and methacrylate esters such as Loctite® formulations262, 271, 290, 609 and 680.

FIGS. 7C and 7D show Assemblages 730 and 740, respectively, eachcomprising multiple Densified Portions 530 arranged such that thelongitudinal direction of the nanotubes is substantially parallel foreach Densified Portion 530. Assemblages 730 and 740 can be created bystacking Densified Portions 530 as shown. Assemblage 740 furthercomprises a Bonding Agent 750 such as adhesive or glue, while Assemblage730 does not.

Any of the Assemblages 710-740 can additionally be further densified,e.g. by the application of a compressive force. Additionally, any of theAssemblages 710-740 can be trimmed by cutting or machining to a desireddimension or shape. Assemblages 710-740 can also be further joinedtogether to form still larger freestanding structures comprisedessentially of only nanotubes. Further still, any of the Assemblages710-740 can be laminated with layers of other materials. For example,ceramic or metallic layers may be combined with nanotube layers foradded stiffness.

FIGS. 8A and 8B illustrate another exemplary method for creating anassemblage. In FIG. 8A an active surface of a substrate is patterned andused to grow parallel Arrays 810 of generally vertically alignednanotubes. Here, a Length 250 for each Array 810 is chosen to be aboutequal to the expected Height 315 of the Arrays 810. The final lateraldimension of the Arrays 810 on the surface (i.e. perpendicular to Length222 and Height 314) can be made arbitrarily large. While the Arrays 810are still attached to the active surface, the Arrays 810 can be wettedand dried wet, resulting in a structure as depicted in FIG. 8B, in whichthe Arrays 810 form Rows 820 of at least partially densified nanotubes.The Rows 820 can be mechanically separated from the substrate andfurther densified according to the method illustrated with respect toFIG. 4B, for example. The Rows 820 can also be at least partiallyseparated from the substrate by etching prior to mechanical compressionas discussed with respect to FIG. 4A.

Although several aspects of the invention address the densification ofan as-grown nanotube array, for some applications it may also beadvantageous to increase the densities of the individual nanotubeswithin an array and/or increase the number of nanotubes per unit area inthe as-grown array. The density of each nanotube (essentially a functionof the nanotube wall thickness), and the number of nanotubes per unitarea in the as-grown array can both be sensitive to several factorsincluding the catalyst composition, distribution on the active surface,and the growth environment. FIG. 9 provides a graphical representationof how nanotube diameter, nanotube wall thickness, and nanotube densitywithin an array can vary for four exemplary catalysts.

As noted above, the invention also includes freestanding objectscomprised of aligned and densified nanotubes. In some embodiments, suchfreestanding objects have a volume of greater than 5 cubic millimetersand comprise at least 10% nanotubes by mass. In further embodiments, afreestanding object can comprise at least 60% nanotubes by mass, andeven at least 90% nanotubes by mass. In some of these embodiments, theobjects have a density greater than 0.4 grams/cc.

In some aspects, the invention provides for nanotube structurescomprising nanotubes, preferably carbon nanotubes, that have axial andradial directions, and by geometry have different properties in theaxial and the radial directions. This property, in some instances,differentiates carbon nanotubes from graphitic sheets that generally donot form tubular structures.

As described above, the invention provides for growing a network ofvertically aligned carbon nanotubes on a substrate which aresubsequently made denser through post-processing to form nanotubestructures, sometimes referred herein as mesh, meshes, sheet, sheets,film, or films. The invention provides for the use of mechanical forceto compress the carbon nanotubes in both parallel and perpendiculardirections relative to the carbon nanotube growth direction. Furtherdensification can be achieved through the use of a liquid. Not wishingto be bound by theory, the inventors believe that the liquid draws thecarbon nanotubes together through capillary forces.

FIGS. 10A-D are scanning electron microscope images of as-grownvertically aligned nanotubes at magnification levels of 5,000×, 10,000×,50,000× and 100,000×, respectively. FIGS. 11A-D are scanning electronmicroscope images of nanotubes after densification via capillary forcesat the same magnification levels as the nanotubes of FIGS. 10A-D, andFIGS. 12A-D are scanning electron microscope images of nanotubes furthercompressed with mechanical force at the same magnification levels. Thedensity of the carbon nanotubes increased around 16 times aftersubsequent exposure to liquid, the increased density likely being due tocapillary forces. An additional four-fold increase in density wasachieved from the application of an additional compressive force afterexposure to liquid and the first compressive step. After densification,the sample was about 65% dense.

As described above, certain aspects of the invention rely, in part, oncapillary and mechanical forces to pull together carbon nanotubes indirections generally normal to the length of the tubes(“x-y-compression”). Mechanical pressure can also be applied in adirection parallel to the direction of carbon nanotube growth(“z-compression”).

In certain embodiments, mechanical force alone, that is, withoutcapillary forces, can produce very dense samples. By way of non-limitingexample, pressing an array of nanotubes that is approximately 3 cm² inarea and with a height ranging from several hundred micrometers toseveral millimeters with several tons of force in the z-direction canyield the mesh of the invention in certain circumstances describedherein.

Certain aspects of this description refer to nanotubes as being“vertically aligned.” As used in this description, that term refers toan alignment generally in a direction normal to the substrate on whichthe nanotubes are grown. It should be understood that the carbonnanotubes described herein are not necessarily perfectly straight andare generally somewhat twisted and entangled with neighboring carbonnanotubes. Without application of the compressive and capillary forcesdescribed herein, the uncompressed nanotube growth process producestypically produces films that are approximately 1% dense, compared tosolid graphite. After compression, such films may retain the originalarea of the uncompressed sample but are much thinner than their originalform. In certain embodiments, the films of the invention range from20-50 μm in thickness.

In certain embodiments, compression may be performed without firstseparating the nanotubes from the substrate. For example, the step ofremoval prior to compression may be omitted if z-compression isperformed while the nanotube array is attached to the substrate. Theresulting film may be removed intact. A liquid release agent, forexample, a solvent such as isopropyl alcohol, may be applied if the filmdoes not readily separate from the substrate. Use of the liquid oftenallows the film to be slid off the substrate with little effort. Otherrelease agents may include ethanol, methanol, acetone, xylene, andwater. In certain embodiments, a separating implement, for example, arazor blade, may be applied between the outer edge of the film and thesubstrate to initiate separation of the film from the substrate.

Mechanical force may be applied in multiple directions simultaneously orsequentially in combination to shape a nanotube structure. As those ofskill in the art will realize, when a material is compressed in onedirection, the other dimensions of the material may increase. Forexample, a compressed nanotube film may be far thinner than itsuncompressed form, but it may also be considerably longer and wider. Formany applications, this dimensional expansion in the uncompresseddirections is acceptable, and may even be desired. However, in someembodiments, it may be helpful to constrain the nanotubes (with grips,plates, barriers, etc.) in the uncompressed direction or directionsprior to compression so as to maintain particular dimensions.Additionally, constraining the nanotubes in particular directions mayprevent the nanotubes from warping out-of-plane during compression.

For example, FIG. 13 is a schematic side elevational view illustratingthe application of mechanical forces in several directions. A nanotubesample 900, which may, for example, be on the order of 1.5 cm×2.0 cm,has been detached from its growth substrate in the illustration of FIG.13. However, in other embodiments, the sample 900 may be attached to itsgrowth substrate. A plate 902 is placed overtop of the sample 900 todistribute force evenly, and force is applied to the plate. However, inFIG. 13, the plate 902 and applied z-direction force is not necessarilyintended to permanently compress or deform the sample 900. Instead, atleast in the arrangement shown in FIG. 13, the z-direction force isapplied at a force level sufficient to constrain the sample 900 andprevent it from warping out of plane as the sample is compressed fromthe other directions. For a small sample 900 of the dimensions givenabove, the z-direction force may be on the order of a few hundred grams,for example, supplied by two 200 g weights resting on the plate 902. Inorder to prevent the plate 902 and its weight from compressing thesample 902, a pair of spacers 904, one on each side of the sample 900,at least partially supports the plate 902. The height of the spacers 904and the amount of weight on the plate 902 may also be chosen to providea defined, limited amount of compression. It may be helpful in someembodiments if the plate 902 is generally smooth and planar, althoughthat need not be so, and in some embodiments, which will be describedbelow in more detail, the plate may have a defined shape or profile ofits own. As one example, a standard laboratory glass slide may be usedas the plate 902 and feeler gages or shim stock of appropriatethicknesses may be used as the spacers 904.

As is also shown in FIG. 13, a second pair of movable members 906 ispositioned inwardly of the spacers 904 and immediately proximate to thesample 902. These movable blocks or members 906, which may have a heightthat is about the same as the height of the spacers 904, bear againstthe sample 900 to apply compressive forces in the x-y plane. To applyx-y compressive forces, the movable members 906 are pushed against thesides of the sample 900. The movable members 906 may be driven manuallyin some embodiments, or they may be part of a load frame that isautomatically driven. Although shown as adjacent in FIG. 13, the spacers904 and movable members 906 may instead be provided adjacent todifferent faces of the sample 900. The final dimensions of the sample900 with the arrangement of FIG. 13 will depend on the degree of appliedforce.

Force application on a nanotube sample may be controlled by controllingthe applied force or by controlling the resulting dimensions of thesample. In many cases, it may be most advantageous simply to decide onthe desired final dimensions of the nanotubes and then to apply forceuntil those dimensions are reached. In general, it has been found thatcompression factors of twenty to twenty-five times the uncompresseddimensions are possible, depending on the density of the uncompressedsample, which may be as little as 1% dense.

The description above generally relates to compression using planar(i.e., flat) plates and other forms. However, the plates, mandrels, andother forms used to compress nanotubes need not be planar. Instead,forms may be curved or have any desired shape.

Additionally, compression may be applied selectively over only a portionof the surface of a sample of nanotubes. This can be particularlyhelpful in creating nanotube structures whose faces have differentshapes, or in creating nanotube structures that have greater outersurface area. Various processes may be used in which force is appliedselectively over only a portion of a nanotube sample. One particularprocess that may be used is stamping.

FIG. 14 illustrates a stamping process. In FIG. 14, a stamp 950 withsome surface features 952 is pressed into a nanotube sample 954 to forma pattern in the sample 954. Similar to the arrangement of FIG. 13,spacers 956 may be provided adjacent to the nanotube sample 954 so as tocontrol the depth to which the stamp 950 penetrates or impresses itselfupon the nanotube sample 954. Of course, as can be seen from the figure,if the height of the nanotube sample 954 is greater than the height ofthe stamp 950, some general compression will occur as the stamp 950 isimpressed. In the illustration of FIG. 14, the nanotube sample 954 isbeing impressed with the stamp 950 on its original growth substrate 958.After impressing, the sample 954 may be removed from the substrate 958by any of the methods described above.

Stamps can be used to impress various types of patterns into samples954. FIGS. 15-17 illustrate various stamp patterns that may be used toimpress nanotube samples 954. In FIGS. 15-17, the heavily shaded areasrepresent depressed areas, while the lightly shaded areas representraised areas. The pattern of FIG. 15 represents a “pin” stamp 970 with anumber of protruding rectilinear “towers.” This type of stamp produces asample with a number of square depressions or holes with a depth thatdepends on the height of the stamp and the placement of spacers 956, ifany. FIG. 16 is a “grid” stamp 972 that is essentially the negative ofthe “pin” stamp 970. The grid stamp 972 produces a sample with a numberof projecting towers. Finally, FIG. 17 illustrates a “slot” stamp 974that produces a series of slotlike depressions in a nanotube sample 954.It should be understood that although the faces of the stamps 970, 972,974 that are impressed into the nanotube sample 954 in the illustratedembodiment are flat, stamps according to other embodiments may havecontoured or rounded faces or shapes and need not be flat orrectilinear.

Although essentially any pattern may be impressed into a nanotube sample954, one consideration when selecting stamp shapes and profiles is thedegree to which the stamp may stick to the sample 954 or otherwise provedifficult to remove once an impression has been made. In someembodiments, a stamp like stamp 970 has been found to be more removablethan stamps 972, 974 with other profiles. However, the degree ofremovability of any particular stamp after an impression has been madewill depend on the size of the stamp, the density and height of thenanotube sample 954, the amount of compressive force applied and/or theoverall amount of compression, among other factors.

Once a pattern has been impressed into the sample 954, it may or may notbe separated from its growth substrate 958. After a pattern has beenimpressed, the sample 954 may be subjected to coating or deposition ofother materials. Certain deposition and coating techniquespreferentially add material to the carbon nanotube structure at theexposed surfaces of the carbon nanotube structure. With such techniques,very little material actually penetrates the network of nanotubes,regardless of whether the network is “as-grown” or has been made denseralready. Therefore, the increased macroscopic surface area provided bythe pattern may be particularly advantageous if the sample 954 is to besubjected to coating or deposition processes that deposit material onthe outer surface of the sample 954. If the sample 954 is to be furtherprocessed or coated, it may be advantageous to leave the sample 954 onits substrate 958 in order to facilitate further processing.

The following examples may illustrate certain aspects of the invention.

EXAMPLES Example 1 Growth of vertically aligned Nanotube Structures

A four inch single crystal silicon substrate was cleaned by immersion ina 4:1 bath of H₂SO₄/H₂O₂, maintained at 120° C., for 10 minutes. Thesubstrate was then rinsed in water and immersed in a 5:1:1 bath ofH₂O/H₂O₂/HCl, maintained at 90° C., for 10 minutes. The substrate wasthen rinsed in water and immersed in a 50:1 HF:H₂O bath, at roomtemperature, for 1 minute. The substrate was then rinsed in water andspun dry.

500 nm of SiO₂ were thermally grown on the cleaned substrate followed by20 nm Al₂O₃ deposited by sputtering from an Al₂O₃ target. Straight, 12.5μm wide boundaries, separating regions to become active surface, werefabricated using lithography, such that each region was a 1 mm widestrip that traversed the length of the wafer. 1 nm of Fe was thendeposited on these regions using electron beam deposition, and then thephotoresist mask was lifted off.

The wafer was then coated with a protective photoresist layer fordicing. The wafer was diced in directions parallel (at 15 mm intervals)and perpendicular (at 20 mm intervals) to the boundaries, such that eachdie had approximately 14 rectangular regions of active surface, eachregion being 20 mm (the die length) by 1 mm (as defined by thelithography boundaries). The photoresist was then stripped and the dieswere cleaned.

Arrays of carbon nanotubes were grown in the regions at 800° C. for 30minutes under conditions previously described. The arrays of nanotubeswere separated in-situ, by etching the interface between the nanotubesand the substrate using the etching procedure previously described.

The density of a representative undensified (as-grown and as-separated)portion of an array was determined by measuring the physical dimensionsof the portion using scanning electron microscopy and weighing thesample. As-grown densities were typically 0.01-0.02 g/cc.

Several portions were exposed to a wetting environment by gently pushingthe portions off of the substrate and into an isopropanol bath. Theresulting wet portions were captured on a glass cover slip substratesuch that the alignment direction of the nanotubes was parallel to thesurface of the substrate. The ends of each portion were affixed using asmall weight to prevent warping during drying. Each wet portion wasallowed to dry at ambient temperature and pressure. Density measurementsof 16 portions subsequent to drying resulted in an average density of0.4 g/cc.

Example 2 Compressing Nanotube Structures

A sample grown using the procedure of Example 1 is placed in an arborpress (e.g. DEVIN LP-500(TM)) and 1 ton of force applied through apolished stainless steel plate placed on top of the sample. Force isapplied in a direction parallel to the growth direction.

A sample is grown using the procedure of Example 1. To establish aninitial height, the sample is first z-pressed with an arbor press usingmetal spacers. With the spacers still in place, a glass slide is thenplaced over the film with two 200 g weights over the slide. The spacersare then slid in toward each other and the film is subsequently pressedin the x-direction. Further compression in the z-direction can beperformed by changing the spacer heights and using the arbor press.

While the invention has been described with respect to certainembodiments and examples, the description is intended to be exemplary,instead of limiting. Modifications and changes may be made within thescope of the invention, which is defined by the appended claims.

1. A method for making a nanotube structure comprising: formingnanotubes such that they are associated with a surface of a substrate;and applying a compressive force to at least a portion of the nanotubesin at least one direction.
 2. The method of claim 1, wherein thenanotubes comprise carbon nanotubes.
 3. The method of claim 1, furthercomprising applying a catalyst onto the surface prior to the nanotubeforming.
 4. The method of claim 1, wherein the compressive force isapplied in a dimension normal or about normal to the substrate surface.5. The method of claim 1, wherein the compressive force is applied in adirection parallel or about parallel to the substrate surface.
 6. Themethod of claim 5, wherein the compressive force causes at least aportion of the nanotubes to dissociate from the substrate surface. 7.The method of claim 1, wherein the compressive force comprises pluralityof compressive forces, each applied from a different direction relativeto the substrate surface.
 8. The method of claim 1, further comprisingdislodging the nanotube structure from the surface using a solvent. 9.The method of claim 1, wherein applying a compressive force to at leasta portion of the nanotubes comprises applying the compressive force tosubstantially the entirety of a surface of the nanotubes.
 10. The methodof claim 1, further comprising constraining the nanotubes in at leastone direction while applying the compressive force in another direction.11. The method of claim 1, wherein applying the compressive force to atleast a portion of the nanotubes comprises applying the compressiveforce selectively to the nanotubes to create one or more surfacefeatures on a surface of the nanotubes.
 12. A method for making ananotube structure comprising: forming nanotubes such that they areassociated with a surface of a substrate; impressing a stamp having astamp surface upon at least a portion of the nanotubes to make animpression in at least the portion of the nanotubes; and removing thestamp from the portion of the nanotubes; wherein the portion of thenanotubes retains at least a portion of the impression.
 13. The methodof claim 12, wherein the stamp has one or more surface features, and theimpression comprises at least a partial impression of one of the one ormore surface features.
 14. The method of claim 13, wherein the nanotubesare substantially aligned with respect to one another and with respectto the surface of the substrate.
 15. The method of claim 14, wherein thenanotubes extend in a direction essentially normal to the surface of thesubstrate.
 16. The method of claim 12, further comprising, before theimpressing, removing at least a portion of the nanotubes from thesurface of the substrate.
 17. The method of claim 12, wherein theimpressing is performed with the nanotubes at least partially attachedto the surface.
 18. A nanotube structure produced by a processcomprising: forming nanotubes on a surface of a substrate; and applyinga compressive force to at least a portion of the nanotubes to form thenanotube structure; wherein the nanotube structure is denser and smallerin at least one dimension than the nanotubes.
 19. The nanotube structureof claim 18, wherein the nanotube structure is produced by a processfurther comprising applying a second compressive force in a seconddirection, and the nanotube structure is denser and smaller in at leasttwo dimensions than the nanotubes.
 20. The nanotube structure of claim18, wherein the nanotube structure comprises two or more connectednanotube structures, each one denser and smaller in at least onedimension than the nanotubes from which it was formed.
 21. The nanotubestructure of claim 18, wherein the nanotube structure is formed by aprocess comprising applying a compressive force selectively to portionsof the nanotubes, such that the nanotube structure has an impressed orembossed pattern on at least one surface.